1. Field of the Invention
This invention relates to magnetic implements for cores of transformers and inductors; and more particularly, to a magnetic implement composed of metal alloy ribbon coated with an electrical insulating material and adapted for use in transformer and inductor cores that operate at high frequencies.
2. Description of the Related Art
Magnetic cores are widely used for electric power transformers that carry electrical current at low frequencies, typically less than 500 Hz. When a transformer is operated at a given frequency, it loses energy, owing in part to its magnetic loss. There are two sources for the magnetic loss: hysteresis loss, which is proportional to the frequency, and eddy-current loss, which is proportional to the n-th power of the frequency where n ranges from about 1.3 to 2. Generally, the proportionality coefficient for the eddy-current loss is proportional to the square of the core material thickness. Thus, electrically insulated, thinner magnetic materials result in lower eddy-current losses. A similar situation exists for inductors operated under an AC excitation. When the operating frequency for these transformers and inductors is low, as is the case for electric power transformers normally operated at 50 or 60 Hz, the magnetic loss due to eddy-current is relatively low compared with hysteresis loss, and some thin oxide coating on the surface of the magnetic material is sufficient to insulate the material. Magnesium oxide is widely used for the coating. However, the degree of adhesion of this oxide to the surface of a magnetic material is poor because it is in a powder form, and the powder often falls off the surface during core production.
Magnetic materials often used as core materials come in ribbon or sheet forms, which are convenient to form tape-wound or stacked cores. Recent advances made in forming magnetic ribbons and sheets include amorphous materials. These materials are conveniently produced using the teachings of U.S. Reissue Pat. No. RE 32925.
When a magnetic core is used at high frequencies, a better or more secure surface coating becomes necessary to ascertain effective electrical insulation between magnetic materials.
A thin, non-conductive insulator, such as paper, polymer film and the like, has been commonly used for such purposes. To maximize the insulation, sufficient dielectric properties are needed for the insulator materials. Low dielectric constants and high dielectric breakdown voltages are generally preferred. These features become increasingly important when the magnetic components are operated at high frequencies and high voltages. Although the magnetic metal-insulator-magnetic metal configuration just described is widely used, the process for manufacturing that configuration has many problems. Such a manufacturing process requires a special apparatus, in which a magnetic metal and an insulator in ribbon or sheet form are co-wound to produce a magnetic component having the form of a wound core. The magnetic metal-insulator-magnetic metal configuration can also be produced by an apparatus wherein a magnetic metal and an insulator are alternately juxtaposed to form a stacked core. Co-winding processes of the alternately juxtaposed type described often result in punctured or torn insulators because of the sharpness of the magnetic metal ribbon or sheet.
There is a need for a method and a means for producing magnetic implements suited for use in transformer and inductor cores that operate at high frequencies. Especially needed is a process for manufacture of magnetic implements comprising amorphous metal ribbon or sheet, which avoids puncturing and tearing of insulators during co-winding operations. A thermally insulated magnetic implement having improved combinations of magnetic properties induced by heat treatment is also needed.